Heat-sensitive recording materials have the merits in that color images can be obtained only by heating, and recording devices are relatively simple and compact, and they are widely utilized as recording papers for various information. Recently, especially heat-sensitive facsimiles and printers are improved to make it possible to carry out high-speed recording which has been difficult to perform. With such increase in recording speed in the field of devices and hardwares, the heat-sensitive recording materials used are required to have improved recording sensitivity, and various proposals have been made therefor. These heat-sensitive recording materials generally comprise a support such as paper, plastic film, or synthetic paper and, provided thereon, a color forming layer of single- or multi-layer mainly composed of adhesives and heat color-forming materials.
For obtaining heat-sensitive recording materials high in sensitivity and high in quality of images formed thereon, there have been proposed to provide an intermediate layer comprising a oil-absorbing pigment to form voids (JP-A-59-155097, JP-A-61-44683) and to employ an intermediate layer of multi-layer to improve smoothness and heat insulation (JP-A-61-11286, JP-A-61-193880).
Furthermore, a recording sheet has been proposed which has an intermediate layer comprising fine hollow particles in addition to the oil-absorbing pigment (JP-A-59-5093). The intermediate layer mainly composed of these organic or inorganic pigments has a high effect, and especially when fine hollow particles are used, since they are lower in thermal conductivity than the generally used oil-absorbing inorganic pigments, very high print image reproducibility can be obtained, but no comprehensive research on binders used for binding the intermediate layer and the support has been made, and, as a result, no satisfactory intermediate layer has been obtained. For example, it has been shown to use water-soluble polymers such as starch, casein, PVA, methylcellulose, carboxymethylcellulose, hydroxyethylcellulose and polyacrylic acid, and various synthetic resin emulsions such as styrene-butadiene copolymer, acrylonitrile-butadiene copolymer, colloidal silica particles-containing styrene-acrylate copolymer and acrylic acid copolymer. However, when an intermediate layer is formed using one or more of these binders, a part of the voids in the resulting intermediate layer are filled up with the binder, and heat insulation is damaged and no sufficient effect to increase sensitivity can be obtained. As a result, heat-sensitive recording materials provided with a high print image reproducibility have not yet been obtained.
JP-A-9-25454 discloses gelled latices for coating, but when they are used together with pigments, gelation occurs or properties of coating solution such as viscosity is unstable and they cannot be stably used.
Further methods have been proposed which include forming on a support a layer comprising foaming plastic fillers (JP-A-59-225987) or thermally expansible fine hollow particles and further providing thereon a layer comprising organic or inorganic pigments (JP-A-64-8088). However, though multilayer coating of the intermediate layer improves thermal recording sensitivity, this is not suitable for production methods of low cost and much consumption type from the point of operability.
Furthermore, attempts to improve contact with thermal head and increase recording sensitivity have been made by enhancing smoothness by cast coating the intermediate layer (JP-A-54-83841, JP-A-56-21889) or by improving smoothness by supercalendering (JP-B-56-86792). However, cast-coating is not suitable for production methods of low cost and much consumption type and supercalendering causes rupture of voids in the layer to damage the heat insulation required for intermediate layer.
Furthermore, for the production of intermediate layer, use of coating solution containing non-crosslinking type alkaline acrylic thickening agent in the case of using calcined clay as pigment has been proposed (JP-A-2-69287). However, pH of coating solutions containing calcined clay or amorphous silica is in the weak acidic region and when an alkali thickening agent is added as it is, action of thickening agent is not sufficiently exhibited and viscosity of the solution becomes unstable. Therefore, in the case of blade coating, pressure applied by the blade varies to cause change in coating amount or formation of streaks on the coating layer. Thus, coating solutions suitable for blade coating can hardly be obtained and, besides, the inherent effect of the intermediate layer cannot be sufficiently obtained.
Generally, an overcoat layer is provided for preventing color formation due to rubbing with fingers (scratch resistance) or decoloration of the images due to solvent or the like. However, when such overcoat layer is provided, the effect of the intermediate layer is not sufficiently exhibited.
The object of the present invention is to provide a method for producing a heat-sensitive recording material which provides a recording material high in sensitivity and whiteness, having high print image reproducibility and less in formation of detritus at the time of recording and, furthermore, a method for producing a high-quality and inexpensive heat-sensitive recording material having an intermediate layer superior in film properties which is less in formation of streak at the time of coating, less in change in viscosity characteristics of coating solution and can assure stable operation.